The invention relates to a method of recovering heavy metals, such as nickel, cobalt, copper, silver, gold, tin, zinc and chromium, from filter residues containing metal oxides, metal hydroxides or metal carbonates, in particular from sludges obtained by the treatment of galvanic waste waters with bases, wherein the residues are dried and reduced during heat supply in the presence of carbon-containing reducing agents and, if desired, upon the addition of additives.
In galavanic plants large amounts of sludge substantially containing one metal only result from galvanic waste waters on individual production sites. Thin sludge usually is obtained by the neutralization of the acid waste waters with basic compounds, such as, e.g., NaOH, Ca(OH).sub.2, CaCO.sub.3, etc., and is further dehydrated to a compact sludge or filter cake by filter presses, decanters and the like. At present, these residues primarily are stored in waste dumps or are destroyed at high costs.
Metal recovering methods are already known, in which metal-oxide containing dusts together with fine-particle carbon-containing reducing agents are heated, thus being reduced in electric or drum furnaces, with CO and/or CO.sub.2 being released. Disadvantages involved therein are working and environmental problems caused by dusting, losses of the metal to be recovered also being unavoidable.
It is furthermore known to pelletize or briquet such metal-containing dusts after the addition of reducing agents, the material to be reduced being guided on a flat bed through a heated rotary hearth like in German Offenlegungsschrift No. 21 34 116. With this method, the temperature is kept below the melting temperature of the material to be recovered, so that a sintered product will result which must be processed further.